Lubricants

A Lubricant is any greasy substance which removes friction between moving parts. Within an engine a lubricant must be clean, so it doesn’t leave deposits which impede performance, durable, and have a high thermal tolerance. Lubricants are sometimes referred to as engine oil, although such products are generally highly engineered. Lubricants generally include products made for gear transmission, axle greasing, ignition systems and combustion chambers.

Lubricants have an increasingly important role in truck operation today as they can improve fuel economy, reduce the downtime and whole life costs of the engine, and they must also be as environmentally sustainable as possible, with longer drain intervals, minimal emissions and the capacity to be recovered and, if possible, recycled.

Emissions technology has seen particulate filters fitted on many trucks and traditional lubricants can clog these over time because of the metal particulates they emit. This can cause a build up of engine pressure and lower performance or break-down. Current research therefore tends to look at the optimal base oils to meet these new operating conditions. This has led to the development of Low-SAPS products – usually synthetics – which contain lower levels of sulphate ash, phosphorus and sulphur.

What do lubricants do?

Lubricants reduce friction, seal the high-pressure combustion gases inside the cylinders, impede the corrosion of metal parts, and transfer heat around the engine. Some lubricants contain friction modifiers which chemically bind to metal surfaces to protect moving parts even where there is too thin a film to provide a physical barrier.

What kinds of lubricants are there?

Different lubricants are suited to gasoline and diesel engines as the load, cycles and fuels are different. Fuels themselves have different inherent levels of lubricity – diesel has higher lubricity than petrol for instance, so tends to give better lubrication during cold starts. Sulphur-based compounds tend to promote lubricity, so Ultra low sulphur diesel (ULSD) has less lubricity than standard DERV (however, it should be noted that sulphur is also potentially corrosive). Biodiesel has a high level of natural lubricity.

There are various types of lubricants for diesel engines, each with different properties:

• Mineral: a petroleum fraction, mineral oil forms the basis of most traditional lubricants, which are typically 90% base oil and 10% additives which improve viscosity, longevity and anti-corrosive capacity. Mineral base oils are usually from Group I or II Saturates.
• Semi-synthetic: It is possible to combine base oils, both mineral and synthetic, such as fluorocarbons and silicones, to produce a semi-synthetic lubricant with the properties of both.
• Synthetic: Group III, (saturates) IV (polyalphaolefins) and V (esters) make up the basis of synthetic oils. Synthetic oils are more expensive than mineral oils but manufacturers claim that they improve fuel economy, extend engine life and have much longer drain intervals.
• Additives: Some of the additives used in lubricant include:
  • Antioxidants 
  • Detergents
  • Anti-wear
  • Metal deactivators
  • Corrosion inhibitors
  • Rust inhibitors
  • Friction modifiers 
  • And additives to improve pressure handling and vicosity, and anti-foaming agents.

Standards for lubricants

The technical specification for lubricants and service intervals can be found at the European Automotive Manufacturers Association. A guide to grades of engine oil can be found at Drivers’ Technology or here.

Real-world view

Prestons of Potto buys its lubricants based on price and service but it says that operators should heed manufacturers’ advice about the type and grade of lubricant to use. Therefore most of its trucks run on semi-synthetics at Scania’s recommendation while its Euro-V Daf takes a synthetic oil because this gives cleaner emissions.

Group II Base Oils

Some manufacturers are introducing lubricants made from Group II base oils as an alternative to synthetics - these are low-SAPs mineral oils, refined using a hydroprocessing technique which converts more than 98% of impurities into high-quality lubricants. The benefits include lower costs than fully synthetics, but with similar high-purity, long-drain properties.

Find out more from this Chevron presentation on Group II base oils.